To print an image on a substrate, a typical “electrostatic” digital printer first forms a copy of the image, conventionally referred to as a “latent image”, on a photosensitive surface of a cylindrical roller, hereinafter referred to as a “photosensitive imaging plate” (PIP). To form the latent image a charger deposits a substantially uniform charge density on the photosensitive surface. The latent image is then formed by a laser that scans the charged photosensitive surface and discharges regions thereon to generate a pattern of charged and discharged or partially discharged pixels on the photosensitive surface that replicates the image to be printed. A developer develops the latent image by applying ink or toner, hereinafter generically, toner, to the photosensitive surface. The toner comprises charged toner particles of a desired color, which in an electrophoretic process migrate and adhere to the charged or the discharged or partially discharged pixels. The toner on the PIP is then optionally transferred from the PIP to a suitable transfer surface of another roller, conventionally referred to as an “intermediate transfer member” (ITM). The toner is transferred from the ITM to the substrate to print the image when the substrate passes through a nip between the ITM and an impression roller. In some cases the toner is transferred directly from the PIC to the substrate without an intermediate transfer via the ITM.
In many printing processes the toner is a liquid toner comprising charged toner particles dispersed in a non-polar carrier liquid having a relatively high electrical resistance. The toner particles generally comprise a polymer in which, optionally, a pigment is dispersed, and a charge control agent, often an organic salt, which provides a net charge in and/or on the toner particles. In order for the toner particles to be properly electrophoretically transferred and adhered to the PIP, the toner particles are charged with an amount of charge that endows them with an appropriate mobility through the carrier liquid, when subjected to an electric field.
Toner particles in a liquid toner are generally charged by adding to the particles at least one charge control agent, alternatively referred to as a “charge director” (CD), which is usually an organic salt. The salt bonds to the toner particles, generally to the polymer in the particles and either an anion or a cation separates from the bonded portion of the salt, leaving the toner particles charged with the charge of the cation or the anion.
A charge priming agent or charge adjuvant (CA), which promotes charging of the toner particles by a charge director is also often added to the particles. For liquid toners, aluminum alkoxide, an oxide such as silica, or a metallic soap, for example, aluminum or magnesium stearate or octoate, are commonly used as a charge adjuvant.
U.S. Pat. Nos. 4,794,651 and 5,565,299 describe materials and processes for preparing a liquid toner. U.S. Pat. Nos. 4,707,429 and 5,225,306 describe materials and processes for preparing a liquid toner using an aluminum stearate charge adjuvant and aluminum alkoxide charge priming agent. U.S. Pat. No. 5,573,882 describes materials and processes for preparing a liquid toner using a charge adjuvant. U.S. Pat. No. 5,393,635 describes a negative charge director for liquid electrographic toners in which a negative charge is generated by a weakly associating charged functional group covalently bonded to the resin of the toner particles and a very strongly chelating molecule dispersed in the liquid phase to achieve charge separation. The disclosures of all the aforementioned US patents are incorporated herein by reference.